java - Using Regex to generate Strings rather than match them

I am writing a Java utility which helps me to generate loads of data for performance testing. It would be really cool to be able to specify a regex for Strings so that my generator spits out things which match this. Is there something out there already baked which I can use to do this? Or is there a library which gets me most of the way there?

Thanks

Answer

Edit:

As mentioned in the comments, there is a library available at Google Code to acheive this:
http://code.google.com/p/xeger

See also https://github.com/mifmif/Generex as suggested by Mifmif

Original message:

Firstly, with a complex enough regexp, i believe this can be impossible. But you should be able to put something together for simple regexps.

If you take a look at the source code of the class java.util.regex.Pattern, you'll see that it uses an internal representation of Node instances. Each of the different pattern components have their own implementation of a Node subclass. These Nodes are organised into a tree.

By producing a visitor that traverses this tree, you should be able to call an overloaded generator method or some kind of Builder that cobbles something together.